Advances in Li–S Batteries

View Online / Journal Homepage / Table of Contents for this issue HIGHLIGHT www.rsc.org/materials | Journal of Materials Chemistry Advances in Li–S batteries Xiulei Ji and Linda F. Nazar* DOI: 10.1039/b925751a Rechargeable Li–S batteries have received ever-increasing attention recently due to their high theoretical specific energy density, which is 3 to 5 times higher than that of Li ion batteries based on intercalation reactions. Li–S batteries may represent a next-generation energy storage system, particularly for large scale applications. The obstacles to realize this high energy density mainly include high internal resistance, self-discharge and rapid capacity fading on cycling. These challenges can be met to a large degree by designing novel sulfur electrodes with ‘‘smart’’ nanostructures. This highlight provides an overview of major developments of positive electrodes based on this concept. Introduction cost factors. These will undoubtedly move the overall redox couple, described by the + À beyond intercalation chemistry into the reaction S8 + 16Li + 16e 4 8Li2S, lies at Following two decades of optimization, realm of ‘‘integration’’ chemistry where an average voltage of 2.15 V with respect Li ion batteries (LIB’s) are approaching discharge/charge of the electrode is to Li+/Li. This potential lies about 1/2 to the energy density limits that intercalation coupled with cleavage/formation of 2/3 of that exhibited by intercalation materials can potentially provide, of covalent bonds together with morpho- positive electrodes. However, this is offset À1 about 300 mA h g . At present, the LIB logical/structural dynamics during redox. by the theoretical specific capacity of 1672 cannot offer a suitably long driving range Examples include the electrochemical mA h gÀ1 afforded by the non-topotactic (i.e., >300 km) for pure electric vehicles reactions that provide the basis for integration process, the highest value for (PEV’s), and it poses limitations for plug- Li–Air1 and Li–S batteries. all known solid cathode materials. Thus, in hybrid electric vehicles (PHEV). Large- Herbet and Ulam first introduced the compared to conventional LIB, Li–S scale, high energy density and inexpensive concept of elemental sulfur as a positive batteries have the opportunity to provide energy storage systems are also required electrode material in 1962.2 Sulfur has a greatly increased energy density at for load leveling for renewable energy many valuable characteristics, such as low a lower cost (see Table 1). Theoretical sources. New systems are being sought for equivalent weight, extremely low cost, and values can approach 2500 W h kgÀ1 or the next-generation batteries to provide nontoxicity. Considerable efforts have 2800 W h lÀ1 on a weight or volume basis, Downloaded by University of Waterloo on 07 March 2012 much higher energy density, and reduce been devoted to alkali metal–sulfur energy respectively,4 assuming complete reaction Published on 10 September 2010 http://pubs.rsc.org | doi:10.1039/B925751A 3 storage systems such as Na–S batteries to Li2S. University of Waterloo, Department of which operate at 300–350 C, and room The earliest configuration of a Li–S Chemistry, WaterlooOntario, Canada N2L temperature Li–S batteries. In a Li–S cell, battery was presented in the late 1960s.5,6 3G1. E-mail: [email protected] Xiulei (David) Ji received his Linda Nazar is Professor of B.Sc. in Chemistry from Jilin Chemistry and Electrical Engi- University in 2003. He obtained neering at the University of his Ph.D. in Materials Chem- Waterloo, Waterloo, Ontario, istry from the University of Canada and holds a Senior Can- Waterloo in 2009, under the ada Research Chair in Solid State supervision of Professor Linda Materials. She received her F. Nazar. He has been awarded Ph.D. in Chemistry from the a Natural Sciences and Engi- University of Toronto, and then neering Canada Postdoctoral joined Exxon Corporate Fellowship that he will take up at Research (USA) as a post- the University of California, doctoral fellow. She was awarded Santa Barbara in 2010. His the Electrochemistry Society Xiulei Ji research interests focus on Linda Nazar Battery Research award in 2009, nanostructured materials and and was the 2010 Moore Distinguished Scholar at the California their applications in energy Institute of Technology. Her research is focused on materials for storage and conversion. energy storage and conversion, with research spanning Li-ion and Na-ion batteries, Li-sulfur and Li-air batteries, and fuel cell catalysts. This journal is ª The Royal Society of Chemistry 2010 J. Mater. Chem., 2010, 20, 9821–9826 | 9821 View Online Table 1 Comparison of a typical C/Li[Co1/3Ni1/3Mn1/3]O2 battery and the Li–S battery Average Theoretical Practical ‘‘Bare-bones’’ energy ‘‘Practical’’ specific discharge capacity of capacity of density of a full energy density of a System potential/V cathode/mA h gÀ1 cathode/mA h gÀ1 cella/W h kgÀ1 full cell/W h kgÀ1 e b C–LiMO2 3.7 275 160 410 135–180 Li–S 2.15 1672 500–1100 950–1700 350c to 700d a Based on active mass (anode + cathode) only—excluding electrolyte, separator and components. b Reported values for full cells from various sources. c d e Data reported by Sion PowerÔ. Based on company projections, and estimated from data in ref. 29. Li[Co1/3Ni1/3Mn1/3]O2. 2À believed to be S8 .The reaction sequence the coulombic efficiency in the charge 13 on reduction and oxidation and the cor- process. It is even observed in Li/TiS2 responding electrochemical profiles are cells.14 On the other hand, the shuttle summarized in Fig. 1. Note that the sulfur provides intrinsic overcharge tolerance active mass (d ¼ 2.03 g cmÀ3) expands for Li–S batteries.15 Mathematical during discharge owing to the lower modeling has yielded a good quantitative À3 16 density of Li2S (1.67 g cm ), and understanding of the phenomenon. It contracts again on charge: an important encompasses theoretical models of the factor to consider when designing charge process, charge and discharge a composite electrode. capacity, thermal effects, self-discharge, Despite the considerable advantages of and a comparison of simulated and Fig. 1 Discharge–charge profiles of a Li–S cell, illustrating regions (I) conversion of solid the Li–S cell, it presents many challenges. experimental data. The study provides 10 sulfur to soluble polysulfides; (II) conversion of The first is the insulating nature of evidence that self-discharge, charge– polysulfides to solid Li2S2; (III) conversion of sulfur and its final discharge products discharge efficiency, and overcharge solid Li2S2 to solid Li2S. which necessitates contact with substan- protection are all facets of the same tial fractions of conductors. The second is phenomenon. Finally, on the completion the solubility of the long chain polysulfide of each discharge process, the soluble 2À The positive electrode comprised ions (Sn ) formed on reduction of S8 or polysulfides are reduced and deposit as elemental sulfur, electronic conductors upon oxidation of the end-member Li2S2/Li2S precipitates on the cathode. (carbon or metal powder) and binders, sulfides, which is especially trouble- Insoluble agglomerates are thus formed separated from the metallic lithium some.11 These molecular species typically on the surface over prolonged cycling negative electrode by an organic electro- diffuse in solution through the separator regardless of the initial cathode 17,18 Downloaded by University of Waterloo on 07 March 2012 lyte. This configuration has been the to the lithium negative electrode where morphology. The agglomerates platform for subsequent major research they are reduced to insoluble Li2Sor become electrochemically inaccessible, Published on 10 September 2010 http://pubs.rsc.org | doi:10.1039/B925751A 8,12 activities as well. In an organic electrolyte, Li2S2. Once the Li anode is fully causing active mass loss at the cathode 2À discharge of the sulfur cell proceeds coated, the following Sn react with these side and the build-up of impedance layers through a three stage process.7,8 In the fully reduced sulfides to form lower order that results in capacity fading. 2À high oxidation state regime correspond- polysulfides (SnÀx ) which become ing to S0 4 S0.5À, sulfur is reduced concentrated at the anode side, diffuse Carbon–sulfur electrodes: through a stepwise sequence of soluble back to the positive electrode and are then overcoming the challenges 2À 2À polysulfide ions to form S4 . The reac- re-oxidized into Sn . The above parasitic tion kinetics are fast owing to the molec- process takes place repeatedly, creating Carbonaceous materials are extensively ular nature of the species involved. The an internal ‘‘shuttle’’ phenomenon. It used as electronic conductors in the second stage corresponding to the reac- decreases the active mass utilization in the battery industry, and play a particularly 0.5À 1À tion of S 4 S to form insoluble Li2S2 discharge process and markedly reduces crucial role in the sulfur electrode. High is hindered by the energy required for carbon content improves conductivity nucleation of the solid state phase. The but at the expense of reduced energy last stage for interconversion of Li2S2 to density. In the earliest cell configura- Li2S is the most difficult. This conversion tions, bulk carbon and sulfur powder is impeded due to the sluggishness of solid were simply mixed together to form state diffusion in the bulk. The charge macrocomposite electrodes. These cells process is much simpler. In cyclic vol- suffered low capacity, and poor cycling tammograms of sulfur or polysulfide life. Peled et al. first described the electrodes, typically there is only one concept of loading sulfur into the porous anodic peak.9 It is suggested that all the structure of carbon materials to establish polysulfides transform into the interme- more efficient electronic contact and diate with the most facile oxidation Fig.

Load more